Optical spectroscopy technology has been widely used to detect, quantify and analyze the characteristics or concentration of a physical, chemical or biological target object such as a blood sample. This technology can also be used in other in vivo chemometric analyses of chemical components of tissues or organs in a living organism. A variety of spectral techniques involve absorption, transmission, reflection, emission, and scattering (including elastic and non-elastic) of radiations applied to a target sample. The radiations used span over a wide range and include UV, Visual, NIR (Near Infrared), SWIR (Short-Wavelength Infrared), MWIR (Medium-Wavelength Infrared), and LWIR (Long-Wavelength Infrared) light.
Optical spectroscopy is also used for highly accurate color measurement of various colored materials. Advanced techniques are used for clinical quantification of blood glucose, dissolved oxygen, dissolved carbon dioxide, urea, lactic acid, creatine, bicarbonate, electrolytes, protein, albumin, cholesterol, triglycerides, bilirubin, heart rate, breathing rate, hematocrit, and hemoglobin.
Optical diagnostics using optical spectroscopy allows for the ability to obtain chemical and biological information without taking a physical specimen, or the ability to obtain information in a non-invasive or non-destructive method from a physical specimen. The challenge is that the adoption of this technology has been limited due to the size of the equipment and the associated cost. Therefore, its application has often been limited to centralized labs with scaled testing protocols. The opportunity now exists to develop a compact and low cost spectrometer. Among those previous efforts to miniaturize the spectrometer to expand the application of optical spectroscopy into broader uses, the planar waveguide-based, grating-based, and Fabry-Perot-based techniques have been the major approaches.
One of the issues encountered when trying to miniaturize the spectrometer is the resolution degradation. The resolution is usually dominated by the optics, especially by the distance from the input slit where the input light comes into the system to the detector array such as a photo diode array (PDA). The shorter the distances, the higher the resolution degradation will be. When filters are used, the number of the filters, and the shape or bandwidth (often measured in terms of FWHM—Full Width Half Maximum) of each filter dominate the degradation. A larger number of filters and a narrower FWHM would provide a higher resolution. However, there is a certain limitation to how narrow the bandwidths of the filters can be, especially when these filters are fabricated in an array configuration.
There are some other issues in using these optical spectroscopy technologies. For example, Berger et al. (U.S. Pat. No. 5,615,673) and Yang et al. (U.S. Pat. No. 6,167,290) each describe a Raman spectroscopic system designed for transdermal analysis of blood components. Xie (U.S. Patent Application Publication No. 2005/0043597) describes a spectral analysis system for analyzing blood components using a radiation passing through a nail of a finger or toe. In these systems, individual variation in skin or nail properties and in blood vessel placement can significantly affect the accuracy of the measurements.